This invention relates to an improved monochromator, and in particular, to a double crystal monochromator in which the surface plane of each of two crystal blocks is inclined with respect to diffraction planes of interest, and more particularly, in which each of two single crystals is cut such that the normal to the diffraction planes of interest makes an angle close to but less than 90 degrees with the crystal surface normal.
The high heat load associated with the powerful and concentrated x-ray beams generated by insertion devices in synchrotron radiation facilities is a very significant factor in the design of monochromators and other optical devices. For example, the undulator source on the Advanced Photon Source ring under construction at Argonne National Laboratory will generate up to 10,000 W of heat on an approximately 1 cm.sup.2 area of the first optics located approximately 24 m from the source. Peak normal flux will be as high as 500 W/mm.sup.2.
For x-ray beams with low to moderate total power and power density, the first of double crystal monochromators may be cooled to reduce the thermal distortions, for example, by pumping a coolant (e.g., water) through appropriately configured cooling channels in the crystal.
The combination of high total power and very high power density found in high power undulator beams is such that conventional cooling of the first crystal as described above is not sufficient to reduce to acceptable levels the high temperatures and resulting thermal strain and stress.
A number of monochromator system designs have been suggested. Broadly categorized, the purposes of these designs are: (a) to reduce the incident power on the first crystal, (b) to reduce the heat flux on the crystal, and (c) to reduce the undesirable misorientation of the diffracted beam due to thermal distortion in the diffraction planes.
Monochromators of type (a) above which are designed to reduce the power of the incident beam may, for example, use filters of the absorbing type (e.g., carbon filters) or use the reflecting properties of a mirror and/or the diffracting properties of a crystal. A transmission x-ray mirror will reflect unwanted low-energy photons while transmitting the desired x-ray pass energy along with all other x rays above the mirror's cutoff energy. (See further, Cowan et al., "Self-Filtering Crystal Monochromators for Synchrotron X Radiation", Rev.Sci.Instrum. (1989) 60(7), 1987).
One way to accomplish purpose (b) above, reducing the heat flux on the crystal, is to increase the area of the beam footprint on the crystal surface by using an asymmetrically cut crystal in which the Bragg diffracting crystal planes are no longer parallel to the surface of the crystal, but make an angle ("cut angle") with the surface. Because the angle of incidence and the angle of exit relative to the surface are unequal, diffraction is also said to be asymmetric. Cutting an asymmetric crystal such that the angle of the diffracting planes relative to the surface is slightly less than the Bragg angle will make the angle of incidence small and the resultant footprint area of the beam on the surface relatively large. (See further, Evans et al., "A `Parallel-Beam` Concentrating Monochromator for X-Rays," Acta Cryst., 1,124, 1948). However, the range in which the monochromator may be tuned is then restricted because the beam must impinge at angles larger than the critical angle for total reflection, where a crystal acts more or less as a mirror. It is possible to tune such a monochromator, however, by rotating the crystal around the surface normal. One disadvantage here is that the position of the footprint on the crystal changes, which, for example, complicates the task of cooling the crystal.
The inclined monochromator of the present invention avoids the problems of asymmetric diffraction while successfully achieving both purposes (b) and (c) above. In addition, it can utilize thermal filters referred to above in (a).
In the inclined monochromator, crystal blocks are cut such that the normal to the diffraction planes of interest makes a prescribed "inclination" angle with the crystal surface normal. For the inclined crystal, both the incident and diffracted beams make the same angle with respect to the surface normal, and, therefore, diffraction is symmetric. Because of the multiplicity of diffraction planes in a crystal, a conventional (symmetrically cut) crystal may be inclined from its conventional setup for the same effect although one is restricted to only a few preset inclination angles. In such cases, diffraction is symmetric regardless of whether the crystal is symmetrically or asymmetrically cut. For example, in a symmetrically cut (111) silcon crystal, there are other (111) planes that are at 70.5 inclination angle with respect to the surface.
The advantages of the inclined monochromator are significant. Using the inclined monochromator the incident beam is spread both vertically and horizontally on the first crystal, providing a larger footprint area and thus reduced heat flux when compared to the conventional monochromator. Because the planes of diffraction are chosen apriori, the monochromator can be operated in a manner identical to that for the conventional monochromator, and the tuning range is practically similar to that of a conventional crystal; Bragg angles at high energies become quite small in both cases, but the critical angle for total external reflection is not reached. An additional, overriding advantage of the inclined monochromator for high heat load applications is that because there is an angle between the surface normal and the diffraction planes only a fraction of the thermal distortion will lie in the diffraction planes, significantly reducing undesirable misorientation of the diffracting planes and thus the diffracting rays from the first crystal of the double-crystal monochromator system.
It is therefore a primary object of this invention to provide a crystal monochromator which under the high heat load associated with recently developed and developing x-ray beams has an acceptable and optimal performance, as measured by the fraction of photons in the desired energy bandwidth that is diffracted from the double-crystal monochromator system and reaches the users' sample.
In the accomplishment of the foregoing object, it is another important object of this invention to provide a monochromator which may be operated with a fixed plane of symmetric diffraction, without sacrificing tunability.
It is another important object of this invention to provide a monochromator which when compared to the conventional monochromator decreases the incident heat flux of the beam by spreading it over a greater area of the crystal surface.
It is a further object of this invention to present a crystal monochromator in which undesirable misorientation of the diffracted rays from the first crystal is reduced by reducing thermal distortion in the diffraction planes.
A yet further object of the present invention is to present an inclined diffraction crystal which provides for a more efficient cooling by spreading the footprint of the incident beam horizontally, so that the footprint becomes very elongated and narrow, and easily cooled, a significant advantage particularly for cryogenic cooling.
Additional objects, advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following and by practice of the invention.